Solid image pickup device, image pickup system and method of driving solid image pickup device

ABSTRACT

The solid image pickup device of the present invention comprises a photoelectric conversion part, a charge-voltage conversion part for converting electric charges from the photoelectric conversion part to voltage signals, a signal amplifier for amplifying the voltage signals generated in the charge-voltage conversion part, charge transfer means for transferring photo-electric charges from the photoelectric conversion part to the charge-voltage conversion part, and means for applying a certain voltage to a charge-voltage conversion part, wherein at least two readout operations for reading out the photo-electric charges accumulated during a period of accumulating photo-electric charges in the photoelectric conversion part via a signal amplifier.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/678,296, filed Oct. 3, 2000 and claims benefit of the filing date ofthat application, and priority benefit of the filing date of Japanesepatent application no. 11-284464 filed Oct. 5, 1999. The entiredisclosures of these prior applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid image pickup device, an imagepickup system and a method of driving the solid image pickup device. Inparticular, the present invention relates to a solid image pickup devicecomprising a photoelectric conversion part, a charge-voltage conversionpart for converting electric charges from the photoelectric conversionpart to voltage signals, signal amplification means for amplifying thevoltage signals, charge transfer means for transferring photo-electriccharges from the photoelectric conversion part to the charge-voltageconversion part, and means for inputting a certain voltage to thecharge-voltage conversion part; an image pickup system; and a method ofdriving the solid image device.

2. Related Background Art

As representatives of solid image pickup devices, there is a devicecomprising a photodiode and a CCD shift register, and a device calledAPS (Active Pixel Sensor), comprising a photodiode and a MOS transistor.

The APS includes a photodiode, a MOS switch, an amplification circuitfor amplifying a signal from a photodiode and the like in each pixel andhas many merits that the “XY addressing”, “making the sensor and thesignal processing circuit into a single chip” or the like is achievable.In recent years, attentions have been attracted to APS owing to apromoted miniaturizing technique of MOS transistors and a raised demandfor “making the sensor and the signal processing circuit into a singlechip” or “reducing the consumption power”.

FIG. 14 shows the pixel part of a conventional ASP and an equivalentcircuit of a solid image pickup device using it. These were reported byMr. Eric R. Fossum et al. at a work shop of IEEE in 1995. Theconfiguration of the prior art will be briefly described below.

The photoelectric conversion part is an embedded-type photodiode (PPD)used in CCD or the like. By providing a concentrated p layer on thesurface, the embedded-type photodiode can suppress the dark currentoccurring at its interface with SiO₂ on it and can provide a junctioncapacity also between the n layer of the accumulation part and the player on the surface of it, thereby increasing the saturated chargequantity of the photodiode.

The photo-signal charges accumulated in the photoelectric conversionpart is read out via the charge transfer means (TX) comprising a MOStransistor to the floating diffusion region (FD).

The signal charges Qsig are voltage-converted into Qsig/CFD by thecapacity of this floating diffusion region (CFD) and the signals areread out through a source follower circuit not shown in FIG. 14.

On applying an inverse bias to the n layer of the embedded-typephotodiode, a depletion layer spreads from individual junctions betweenthe concentrated p layer in the surface and the P well of the substratein accordance with the bias. At this time, the number of electrons inthe photodiode is almost equal to that of the intrinsic regionsandwiched between both depletion layers and decreases in proportion tothe width of the depletion layer. The number of electrons in the aboveintrinsic region at the time of the inverse bias=0 volt corresponds tothe saturated charge quantity. When both depletion layers spread underaction of the inverse bias and are connected to each other, the interiorof the photodiode is depleted and the intrinsic region disappears. Theinverse bias at this time is referred to as “depleted voltage (orcompletely depleted voltage)” hereinafter. Furthermore, when an inversebias is applied increasingly, the electron concentration in thephotodiode exponentially decreases, depending on an increase in inversevoltage. With the above sensor, if the photodiode interior is completelydepleted in readout, charges generated by light are almost completelytransferred to the floating diffusion region and simultaneously thecharges are absent in the photodiode, thereby fulfilling the reset ofelectrons. Hereinafter, such a charge transfer is referred to as“depletion transfer”.

FIG. 15 shows the saturated charge quantity Qsat of a photodiode, avalue of voltage VFDsat ((1) and (2) of FIG. 15) of the floatingdiffusion region in readout of the saturated charge, and the depletingvoltage (3) with respect to the saturated voltage Qsat.

Symbol A denotes the lower limit value of saturated charge required fora practical photodiode, and symbol F denotes the upper limit value ofsaturated charge required for a practical solid image pickup device,while Symbols B and E denote the values of saturated charge quantity atVFDsat=depleted voltage.

VFDsat is given in terms of the following formula.VFDsat=Vres−Qsat/CFD

Vres represents the reset voltage of the floating diffusion region.

In general, the saturated voltage of a photodiode is required to be morethan a certain value and its lower limit value is a value denoted by Ain FIG. 15. Besides, in order to attain the above depletion transfer, itis demanded to actualize the relationship of: VFDsat≧depleting voltage,and preferably

VFDsat>depleting voltage.

Thus, in case of (1) of FIG. 15, the upper limit value of depletedvoltage satisfying this relationship is denoted by B of FIG. 15.

In case of VFDsat<depleted voltage, the inverse bias voltage of aphotodiode becomes equal to VFD, an intrinsic region is present in thephotodiode, and readout is carried out in accordance with the capacitydivision between the capacity by the above depletion layer and thecapacity of the floating diffusion layer. Together with this, even afterthe readout, the amount of residual electrons close to the saturatedcharge quantity Qsat is present and the depletion layer does not occur.The residual electrons of this time causes an afterimage and a noise.

Accordingly, the design of the photodiode is required so that thesaturated charge quantity Qsat in the photodiode meets the range C ofA<Qsat<B.

The saturated charge quantity Qsat, or the depleting voltage, has aproblem, however, likely to be affected by the production process. Forexample, it happens that only a 10% fluctuation of ion implantation dosequantity in the formation of the n layer of a photodiode brings about achange of 0.4 volts in the depleting voltage.

As a result, the yield of production lowers. As one method for avoidingthese problems, the value of reset voltage Vres in the floatingdiffusion region is increased to obtain a state as indicated by thestraight line (2) of FIG. 15, whereby the selection margins of thesaturated charge quantity Qsat can be extended to the range of A-E. Inthis case, a higher reset voltage becomes necessary. This means that ahigh power source voltage must be secured to assure the signal/noiseratio and a great factor of obstructing a lower voltage of the APS liesin this point.

As well known, a high power source voltage brings about a rise in powerconsumption. Besides, in case of being integrated with a logic circuit,another high power source voltage must be prepared for a sensor chipindependently of a low power source voltage of the logic circuit. Thisresults in the deterioration of performances of an APS chip.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a solid imagepickup device having a less amount of noise at a smaller powerconsumption in comparison with a conventional one, a method of drivingthe solid image pickup system, and an image pickup system.

It is another object of the present invention to provide a solid imagepickup device capable of performing depletion transfer without raisingthe power source voltage or the reset voltage, a method of driving thesolid image pickup system, and an image pickup system.

The present invention provides a method of driving a solid image pickupdevice equipped with pixels, each of the pixels comprising aphotoelectric conversion part and output means for outputting signalsfrom the photoelectric conversion part, which method comprises dividingphoto-electric charges accumulated in the photoelectric conversion partduring one unit of accumulation period and reading out the charges viathe output means.

The present invention provides a method for driving a solid image pickupdevice comprising a photoelectric conversion part, a charge-voltageconversion part for converting electric charges from the photoelectricconversion part to voltage signals, signal amplification means foramplifying the voltage signals generated in the charge-voltageconversion part, charge transfer means for transferring photo-electriccharges from the photoelectric conversion part to the charge-voltageconversion part and reset means for resetting the charge-voltageconversion part by applying a predetermined reset voltage thereto, whichmethod comprises, in a readout period of reading out from thephotoelectric conversion part the photo-electric charges accumulated inthe photoelectric conversion part during one unit of accumulationperiod, transferring a part of the photo-electric charges from thephotoelectric conversion part to the charge-voltage conversion part, andperforming a first readout operation of reading out output signalsamplified by the amplification means to a signal output line, and thenresetting the charge-voltage part, transferring the rest of thephoto-electric charges from the photoelectric conversion part to thecharge-voltage conversion part and performing a final readout operationof reading out the output signal amplified by the amplification means tothe signal output line.

Further, the present invention provides a solid image pickup deviceequipped with pixels, each of the pixels comprising a photoelectricconversion part and output means for outputting signals from thephotoelectric conversion part, wherein the device further comprises acircuit for dividing photo-electric charges accumulated in thephotoelectric conversion part during one unit of accumulation period andreading out the charges via the output means.

Furthermore, the present invention provides a solid image pickup devicecomprising a photoelectric conversion part, a charge-voltage conversionpart for converting electric charges from the photoelectric conversionpart to voltage signals, signal amplification means for amplifying thevoltage signals generated in the charge-voltage conversion part, chargetransfer means for transferring photo-electric charges from thephotoelectric conversion part to the charge-voltage conversion part andreset means for resetting the charge-voltage conversion part by applyinga predetermined reset voltage thereto, wherein the device furthercomprises a control circuit for control so as to, in a readout period ofreading out from the photoelectric conversion part the photo-electriccharges accumulated in the photoelectric conversion part during one unitof accumulation period, transfer the photo-electric charges from thephotoelectric conversion part to the charge-voltage conversion part andperform a first readout operation of reading out output signalsamplified by the amplification means to a signal output line, and thento reset the charge-voltage conversion part, transfer the rest of thephoto-electric charges from the photoelectric conversion part to thecharge-voltage conversion part and perform a final readout operation ofreading out the output signal amplified by the amplification means tothe signal output line.

According to the present invention, the photo-electric charges remainingin the photoelectric conversion part can be read out without raising thereset voltage so much by performing a readout operation twice or more inthe case of reading out the photo-electric charges accumulated in thephotoelectric conversion part during one unit of accumulation period.Furthermore, if summing the output signals read out, a wide dynamicrange of signals can be obtained.

And, it is to be noted that the present invention differs from a knownart which comprises reading out the signals accumulated during the firstaccumulation period, entering the second accumulation period, and thenreading out the signals accumulated during the second accumulationperiod and extending the dynamic range by summing those signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D and 1E are schematic illustrations of a pixel partconfiguration of a solid image pickup device according to the presentinvention and its operation;

FIG. 2 is a driving timing chart of the pixel part used for the presentinvention;

FIG. 3 is a representative circuit diagram of the pixel part used forthe present invention;

FIG. 4 is a schematic circuit diagram of a readout circuit with anaddition circuit used for the present invention;

FIG. 5 is an illustration of one example of the addition circuit usedfor the present invention;

FIG. 6 is a circuit diagram of the readout circuit with another additioncircuit used for the present invention;

FIG. 7 is a driving timing chart of the pixel part used for the presentinvention;

FIG. 8 is a circuit diagram of another solid image pickup device usedfor the present invention;

FIG. 9 is a circuit diagram of still another solid image pickup deviceused for the present invention;

FIG. 10 is a graph showing one example of relationship between the lightquantity and an output signal after the calculation;

FIG. 11 is a circuit diagram of another readout circuit used for thepresent invention;

FIG. 12 is a circuit diagram of still another readout circuit used forthe present invention;

FIG. 13 is a block diagram of a image pickup system used for the presentinvention;

FIG. 14 is a schematic illustration of the operation of a conventionalsolid image pickup device; and

FIG. 15 is a graph showing a relationship between the saturated chargequantity of a photodiode and the potential of the floating diffusion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1A to 1E, 2 and 3, the basic operation principle ofthe present invention will be described in further details.

FIGS. 1A to 1E typically show relations between a partial section of asolid image pickup device and its potential for explaining the principleof the present invention.

FIG. 2 is a driving timing chart showing a method of driving a solidimage pickup device according to the present invention.

FIG. 3 is a circuit diagram of one pixel in a solid image pickup deviceaccording to the present invention.

FIG. 1A shows a section of the portion including a photodiode as aphotoelectric conversion part, a transfer gate as charge transfer means,a floating diffusion region as a charge-voltage conversion part(semiconductor diffusion region) and a reset switch as reset means.

Numeral 101 denotes a P well to function also as the anode of thephotodiode; 102: the transfer gate; and 103: the floating diffusionregion comprising n-type semiconductor.

Numeral 104 denotes a surface p region of the photodiode which comprisesa p-type semiconductor and numeral 105 denotes an n region to functionthe cathode of the photodiode which comprises an n-type semiconductor,while an embedded photodiode is formed by the presence of this surface Pregion. Numeral 106 denotes an insulating film made of silicon oxide orthe like and numeral 17 denotes a reset region made of an n-typesemiconductor to which a predetermined reset voltage is applied viawiring or the like, while the reset region is kept at a predeterminedpotential. Numeral 108 denotes a reset gate of the reset switch forresetting the floating diffusion region to a predetermined potential.The n region 105 and the floating diffusion region 103 partly serves fora source and a drain region of the reset switch. The floating diffusionregion 103 and the reset region 107 partly serves for a source and adrain region of the reset switch. The floating diffusion region 103 isconnected to a gate of amplification transistor (not shown) serving forsignal amplifier means and serves for the input part of the signalamplification means.

And, output means is composed of the transfer gate, the floatingdiffusion region, the reset switch and the amplification transistor andused to read out the photo-electric charges (here, electrons)accumulated in the photodiode.

The circuit diagram of one pixel of the solid image pickup device inwhich this configuration is used becomes as shown in FIG. 3, Numeral 505denotes a photodiode and Symbol Q1 denotes a transfer switch; Q2: areset switch; and Q3: an amplification transistor. Symbol Q4 denotes aselection switch not only for selecting a pixel and but also for readingout an output signal current-amplified from the amplificationtransistor.

FIG. 1B shows a state during the transfer of a part of charges at ONstate of the transfer switch, FIG. 1C shows a state immediately afterthe transfer switch turns OFF, FIG. 1D shows a state immediately afterthe reset switch turns OFF after the reset switch turns ON to reset thefloating diffusion region, and FIG. 1E shows a state in which thetransfer switch turns ON again to depletion-transfer the residualcharge.

In the temporal sequence of FIGS. 1A, 1B, 1C, 1D and 1E, the operationproceeds.

The outline of the driving timing including the above operation will bedescribed referring to FIG. 2.

Incidentally, “reset” in FIG. 2 is not limited to a driving pulse to aMOS transistor for resetting and indicates the reset operation ingeneral, while the pulse being at the high (ON) state indicates theexecution of its reset operation. On the other hand, the same is appliedfor “readout” and this “readout” indicates to the readout operation ingeneral and the pulse being at the high (ON) state indicates theexecution of its readout operation.

First, the accumulation of photo-electric charges to a photodiodebegins.

According to the need, the selective switch Q4 turns ON to read out asignal based on the reset voltage of the floating diffusion region 103during the period T0. Since this signal is amplified by means of anamplification transistor, this signal can be regarded as the noisesignal of this pixel.

Next, during the period T1, the transfer switch turns ON as shown inFIG. 1B to transfer a part of the photo-electric charges from the nregion 105 of the photodiode 505 to the floating diffusion region 103midway through the accumulation period.

After the transfer, as shown in FIG. 1C, the rest of the photo-electriccharges remains with a photodiode exposed to intense light near asaturated state. With a photodiode exposed to very weak light, allcharges may be transferred in some cases.

During the period T3, the selective switch Q4 turns ON to read out theoutput signal based on the charge transferred to the floating diffusionregion 103.

And, during the period T4, the reset switch Q2 turns ON to reset thepotential of the floating diffusion region. The state after the reset isshown in FIG. 1D of this pixel.

Furthermore, if necessary, the selective switch Q4 turns ON during theperiod T5 to read out the signal based on the reset voltage of thefloating diffusion region 103. Since this signal is amplified by meansof the amplification transistor, this signal can be regarded as a noisesignal of this pixel.

During the period T6, again, the transfer switch turns ON to put theaccumulation period to end and to transfer the rest of thephoto-electric charges from the n region 105 of the photodiode 505 tothe floating diffusion region 103. The state at this time is shown inFIG. 1E.

Unless the accumulation period is controlled by a shutter such asmechanical shutter outside the solid image pickup device, strictlyspeaking, the accumulation period at the transfer during the period T6becomes longer than that at the transfer during the period T1. Since theaccumulation time (exposure time) prior to the period T1 is sufficientlylong, however, the time from the end of the period T1 to the start ofthe period T6 is one-hundredth of the accumulation time at most andnegligible.

After the completion of the period T6, since the n regions 105 of allphotodiodes 505 are depleted, all photodiodes are reset to the initialstate. At this time, if a photodiode is exposed to light, the nextaccumulation period starts from after the completion of this period T6.

Furthermore, during the period T7, the selective switch Q4 turns ON toread out the signal based on the transfer charge of the floatingdiffusion region 103. And, in a circuit outside the pixel, if necessary,the signal read out during the period T3 and the signal read out duringthe period T7 are summed.

After the completion of the period T7, the reset switch Q2 turns ONagain to reset the potential of the floating diffusion 103.

The operation by this embodiment will be described in details.

Here, referring to FIG. 15 again, the case where the saturated chargeamount Qsat of a photodiode lies between B-F is taken intoconsideration.

In this case, if the reset voltage applied to the floating diffusion 103is raised to maintain the condition of VFDsat>depleting voltage betweenB-F, the depletion transfer can be actualized by one-time transferoperation.

In contrast, if the reset voltage is lowered like the straight line (1)in FIG. 15, a large amount of charges remains in a photodiode even bymeans of a transfer operation because the relation VFDsat<depletingvoltage is maintained according to the relations (1) and (3) in FIG. 15when the transfer switch is opend at a state that charges correspondingto the saturated charge quantity are accumulated in the photodiode (thetransfer switch turns ON with the transfer gate 102 kept at the highlevel during the period T1 of FIG. 2). This situation is just as shownin FIG. 1B.

With a certain method out of the prior art, the transfer switch isclosed at this state (the transfer switch turn OFF with the transfergate 102 kept at the low level as in the period T2 of FIG. 2) to startthe next accumulation. Thus, in a photodiode, at the time of reading outthe signal charges accumulated in the next accumulation period, signalsmixed with the residual charges which have not been completelytransferred at the preceding time generate to result in the occurrenceof an afterimage.

If the method of further lowering the depleting voltage is adopted tosolve this problem, the charge amount allowed to be handled in a solidimage pickup device decreases and the solid image pickup device becomesimpossible in fully exhibiting its performance. Such being the case, thepower source has been forced to be raised to secure the handled chargeamount.

Accordingly, it was difficult to use a fine MOS transistor in the pixelpart and making the APS sensor finer has become difficult.

In the present invention, after the completion of the first signalreadout period T3, resetting is performed with the reset gate 108 keptat the high level during the reset period T4 of FIG. 2. Then, as shownin FIG. 1D, the floating diffusion region 103 serving for the input ofsignal amplification means is once reset. Thereafter, during the periodT6, the transfer switch is opened with transfer gate 102 kept at thehigh level to perform a signal transfer, and residual charges are readout as shown in FIG. 1E then, during the period T7, to read out thesignal portion that could not be read out at the preceding signalreadout. In this manner, the photo-electric charges accumulated in thephotodiode is entirely transferred and the photodiode is completelydepleted.

After this, if resetting the floating diffusion region is carried out,the afterimage to be generated in this accumulation period vanishes.

Besides, if necessary, a third readout operation may be carried outfurther after once resetting the floating diffusion region 103 servingfor the input part of signal amplification means. As a matter of fact, afourth time and further readout operation may be carried out.

Furthermore, on repeating the above readout operation by the times inwhich the charges in the photodiode can be fully read out, the maximumcharge quantity by which the photodiode can be handled without anyafterimaage can be read out.

Besides, in the present invention, by summing the signals from the abovesignal amplification means, e.g. by summing the three-fold outputs fromthe signal amplification means obtained by three-fold readout operationsas mentioned above, a greater amount of photo-electric charge signalscan be also read out.

Though there was formerly a way of thinking to sum the output signals,such a technique was to sum the outputs substantially different inaccumulation time as represented by a photometric sensor of a camera.

Besides, as a technique of summing the signals identical in accumulationtime, a technique of summing the signals of other pixels is referred toas represented by color processing.

In contrast to the above, in the embodiments of the present invention,signals of an identical pixel for one unit of accumulation time isdevided and read to sum the signals.

Several summing means exist and as one example, after digital-convertingan output from the signal amplifying means, an AD converter fordigitally adding and a digital addition circuit can be used. Besides, anaddition circuit for adding outputs of individual times after weightedmay be adopted, and the weighted addition permits the sensitivity andgamma to be varied with different ranges of light quantity. Besides, ananalog adder for the analog addition may be used before the digitalconversion.

Especially in case of using an embedded-type photodiode as thephotodiode, no noise such as reset noise is heaped in every eachtransfer operation, so that the added information is not inferior to theinformation read out by one-time readout with an elevated voltage atall.

This point will be described below in further details.

By the times where all charges in the photodiode can be transferred asmentioned above, the above transfer/readout operation is repeated to addthe obtained signals. Here, it is important that even if read outcharges disperse at the time of individual readout, the readout chargesare added by using an embedded-type photodiode to finally read out allcharges, thereby obtaining the whole charge amount. Accordingly, thenoise due to the division of readout is not included. A specific examplewill be taken in the description. As a result of accumulation, 100charges are accumulated. If 50 charges are read out at the first timeand 40 charges are read out at the second time, only 10 charges remainin an embedded-type photodiode and consequently the third time read outbecomes 10 charges. Thus, the sum results in 100 charges. Principally,the number of charges fluctuates at individual times and there is a casewhere 48 charges and 38 charges are read out at the first and the secondtimes, but in this case only 14 charges remain in the embedded-typephotodiode and consequently the third time readout becomes 14 charges.Thus, the sum results in 100 charges.

As a result, a photodiode has only to satisfy the condition capable oftransferring a desired amount of saturated charge Qsat: Vres>depletingvoltage, and a still lower voltage is executable than in the prior art.

As mentioned above, according to the present invention, readoutoperations of more than twice permits the photo-electric chargesremaining in the photoelectric conversion part to be read out, andfurther by adding the readout signals, a wide dynamic range of lightsignal can be obtained.

As a matter of course, the present invention is effective for either ofa linear sensor with pixels arranged in one dimension and an area sensorwith pixels arranged in two dimension, but in an area sensor having astrong need for pixel reduction, the present invention is moreeffectively used because there are many restrictions for the type andnumber of transistor and no circuitry measure cannot be taken.

Embodiment 1

The equivalent circuit diagram of the pixel used in this embodiment isthe same as that shown in FIG. 3, and in this embodiment this pixel isarranged in two-dimensions to form an area sensor.

In FIG. 3, Numeral 505 denotes an embedded-type photodiode correspondingto the photoelectric conversion part. As an embedded-type photodiode inthis embodiment, an n-type accumulative layer for accumulatingphoto-electric charges and a P⁺-type layer comprising a p-typesemiconductor of a high impurity for suppressing the surface darkcurrent between this n-type layer and an insulating layer thereon wereprovided in a well comprising a p-type semiconductor formed in thesubstrate to form an embedded-type photodiode as shown in FIG. 1. Thedepleting voltage of this photodiode is 1.0 volt.

An nMOS transistor was used for an input transistor of a source followeramplifier as the signal amplifying means Q3, and the nMOS transistor wasused for selecting a reading out row as a selection switch Q4.

Though not shown, a constant-current load is connected to a signaloutput line 504 as the load of the source follower.

As the reset switch Q2 for resetting the input terminal of the sourcefollower, an n MOS transistor was used.

As the transfer switch Q1 for transferring a photo-electric signal ofthe photodiode 505 to the input part, a transfer gate was provided onthe region between the n layer of the photodiode and the floatingdiffusion layer. This transfer gate corresponds to charge transfer meansfor transferring charges to the input part of the source follower as thesignal amplification means.

Numeral 501 denotes a power source line applying a reference voltage forthe reset and for the amplification; 502: a reset switch line forcontrolling the operation of the reset switch Q2; 503: a selectiveswitch for controlling the operation of the selective switch Q4; and506: a transfer switch line for controlling the operation of thetransfer switch.

FIG. 4 is a schematic circuit diagram of the readout circuit equippedwith adder means used in the present invention.

In FIG. 4, Numerals 601 denotes a pixel, as a simplified representationof the pixel shown in FIG. 3.

Output of each individual pixel is read out onto the output signalretention means 602 connected to a signal output line 604. For theoutput signal retention means 602 for once retaining an output signal, aplurality of capacity elements can be specifically used. In thisembodiment, at the primary (first) readout, a switch S1 in FIG. 4 turnsON/OFF to retain an output signal in a capacity. At the secondaryreadout, a switch S2 in FIG. 4 turns ON/OFF to retain an output signalin another capacity. At the final (third) readout, a switch S3 in FIG. 4turns ON/OFF to retain an output signal in yet another capacity.

The respective signals retained in individual capacities are added bythe analog adder as signal addition means 603, converted in time seriessignals by a horizontal scanning circuit 605 and outputted via an outputamplifier 606.

The circuit of FIG. 4 contains no substraction processing means, but canalso be so configured as to include the substraction processing means inthe output signal retention means 602. In this case, signals subjectedto the subtraction processing and freed of noises are added in theoutput signal addition means 603, outputted to an output line by thehorizontal scanning circuit 605 and outputted via the output amplifier606.

As the addition means 603, a clamp type addition circuit as shown inFIG. 5 can also be used.

In FIG. 5, capacity elements (clamp capacities) are connected in seriesto three terminals and an amplification transistor comprising a resetswitch and a source follower is connected to the output side of acapacity element. The output signals that have been read out are addedby three clamp capacities and outputted from the amplificationtransistor.

The outline of the driving timing is as follows.

By the OFF state or with shutter opening of the transfer switch Q1, theexposure to the phototransistor Q1 begins.

After the lapse of a one-field scanning period or a period equivalent toa one frame scanning period, the input part of a source follower isreset by turning the reset switch Q2 from ON to OFF, then and set to afloating state.

Next, the transfer switch Q1 turns ON/OFF to transfer a part ofphoto-electric charges accumulated in the photodiode 505 to the inputpart of the source follower.

Not only by turning the selective switch Q4 of a pixel ON but also byON/OFF of the switch S1, an output signal is retained in the capacity602.

Again, the reset switch Q2 turns ON/OFF and the input part of the sourcefollower is reset and set to a floating state. The transfer switch Q1turns ON/OFF to transfer a part of the rest of photo-electric chargesaccumulated in the photodiode 505 to the input part of the sourcefollower.

Similarly to the above preceding, to obtain a second readout signal thistime, the selective switch Q4 of a pixel turns ON and the switch S1turns ON/OFF to retain an output signal in the capacity 602.

To obtain the third readout signal, the reset switch Q2 turns ON/OFF andthe input part of the source follower is reset and set to a floatingstate. Then, the transfer switch Q1 turns ON/OFF to transfer a part ofthe rest of photo-electric charges accumulated in the photodiode 505 tothe input part of the source follower. And the selective switch Q4 of apixel turns ON, the switch S1 turns ON/OFF to retain an output signal inthe capacity 602.

According to this embodiment, even in case of transferring a signal fromthe photodiode kept in a saturated charge and reading it out, thephotodiode can be completely depleted by three times of the transfer andreadout operation. In a photodiode of a small accumulated charge amount,the charges are completely transferred by the second transfer operationand the photodiode is completely depleted. Furthermore, in somephotodiodes of a still smaller accumulated charge amount, the depletedreset is attained by the first transfer operation.

Formerly, to ensure its output signal amplitude at 2.5 volts, the sourcefollower had to be driven at a power supply voltage=5.0 volts and areset voltage=3.5 volts.

On the other hand, according to this embodiment, in spite of loweringthe power supply voltage and the reset voltage to 3.3 volts and 1.8volts, respectively, good light signal can be obtained which isequivalent to the conventional one.

Besides, to attain the above performance, the power supply voltage isrequired to be 5.0 volts and a 0.8 μm rule MOS transistor process had tobe used formerly, whereas the power supply voltage can be set to 3.3volts and a 0.35 μm rule MOS transistor process can be used according tothis embodiment.

Embodiment 2

The physical configuration of individual pixels and the circuitry of asolid image pickup device according to Embodiment 2 are the same asthose Embodiment 1.

Difference from Embodiment 1 lies in using the circuit shown in FIG. 6as a readout circuit.

FIG. 6 is a schematic circuit diagram of a readout circuit with additionmeans, used in the present invention.

Output of each pixel is read out by the output signal retention means702 connected to a signal output line 704. For output signal retentionmeans 702 for once retaining an output signal, specifically, a pluralityof capacity elements can be used. In case of this embodiment, the switchS1 in FIG. 5 turns ON/OFF at the primary (first) readout to retain anoise signal and an output signal in a capacity. At the second readout,the switch S2 turns ON/OFF to retain a noise signal and an output signalin another capacity, and at the final (third) readout, the switch S3turns ON/OFF to retain a noise signal and an output signal in yetanother capacity.

The respective noise and output signals retained in individualcapacities are converted into time series signals at the horizontalscanning circuit 705 and outputted from the output amplifier 706.

The drive timing chart is shown in FIG. 7.

After turning the reset switch Q2 from ON to OFF, the input part of thesource follower is reset and is set to a floating state. The selectiveswitch Q4 of a pixel turns ON, the reset noise generated by this resetoperation is read out to the output signal line 704 and one of theswitch S1 turns ON/OFF to transfer the noise signal to the capacity forretaining a noise signal. The high level pulse S1 (N) for readout inFIG. 7 is a signal for opening one of the switch S1.

Next, the transfer switch Q1 turns ON/OFF and a light signal istransferred from the photodiode 505 to the input part of the sourcefollower to overlap the light signal component on the reset noiseremaining in the input part. The selective switch Q4 turns ON to readout this light signal onto the output line 704 and retain this signal inanother capacity by opening the. other of the switch S1. The high levelpulse S1 (S) for readout in FIG. 7 is a signal for opening the other ofthe switch S1.

Thereafter, again, the reset switch Q2 turns ON/OFF and the input partof the source follower is reset and set to a floating state. Similarlyto the above preceding, to obtain the second readout signal this time,the switch S2 successively turns ON/OFF by means of high level pulses S2(N) and S2 (S) to retain output signals in respective capacitiestogether with reset noises and light signals.

Furthermore, to obtain the third readout signal, the switch S3successively turns ON/OFF by means of HIGH level pulses S3 (N) and S3(S), similarly to the above, to retain a noise and an output signal inrespective capacities.

In Embodiment 2, the photodiode and the reset voltage are designed so asto be able to transfer the whole charge in an embedded-type photodiodekept in a saturated state or in an accumulated state of the maximumaccumulated charge on the system.

Formerly, to ensure the output signal amplitude of a source follower at2.5 volts, the device had to be driven at a power supply voltage of 5.0volts and a reset voltage of 3.5 volts.

On the other hand, according to this embodiment, in spite of loweringthe power supply voltage and the reset voltage to 3.3 volts and 1.8volts, respectively good light signal can be obtained which isequivalent to the conventional one.

Besides, to attain the above performance, the power supply voltage isrequired to be 5.0 volts and a 0.8 μm rule MOS transistor process had tobe used formerly, whereas the power supply voltage can be set to 3.3volts and a 0.35 μm rule MOS transistor process can be used according tothis embodiment.

Embodiment 3

FIG. 8 is a circuit diagram of one pixel and a readout circuit accordingto the present invention.

Numerals 901 and 901′ denote power supply lines for applying the resetvoltage and the power supply voltage of an amplification transistor.Numerals 902 and 902′ denote reset switch lines for controlling theoperation of the reset switches Q2 and Q2′. Numeral 904 denotes a signaloutput line. Numeral 905 denotes a photodiode. Numerals 906 and 906′denote transfer switch lines for controlling the operation of transferswitches Q1 and Q1′, respectively.

In Embodiments 1 and 2, one source follower was disposed for each pixeland the input part of a source follower was reset in the time series forevery readout.

In this embodiment, two source followers Q3 and Q3′ are disposed foreach pixel. Here, the input parts of source followers Q3 and Q3′ aresimultaneously reset by turning the reset switches Q2 and Q2′ to ON.

Thereafter, the selective switch Q4, the noise switch of the switch S1,the selective switch Q4′ and the noise switch of the switch S2successively turn ON/OFF to retain reset noise signals in the noisecapacities respectively connected to the switches S1 and S2.

Subsequently, the transfer switches Q2 and Q2′ successively orsimultaneously turn ON/OFF to transfer charges to the input parts ofsource followers Q3 and Q3′ and the selective switches Q4 and Q4′successively turn ON/OFF to retain signal outputs containing lightsignal information in capacities respectively connected to the switchesS1 and S2.

And, using a horizontal scanning circuit, signals are outputted whileadded, subjected to noise suppression treatment and outputted.

Embodiment 4

FIG. 9 is a circuit diagram of a solid image pickup device according toEmbodiment 4.

Here, a second signal output line 904′ is disposed in parallel with afirst signal output line 904 and signal outputs containing noise signalsand light signals can be read out in parallel by reading out the outputof a source follower Q3′ onto the second signal output line. Namely, theselective switches Q4 and Q4′ can simultaneously turn ON/OFF.

According to this embodiment, a light signal was obtained at a lowvoltage and in a wide dynamic range similarly as in Embodiment 1 and atthe same time the readout time could be shortened though the pixel sizeincreases as compared with Embodiments 1 and 2.

Embodiment 5

For example, by modifying the readout circuit of a solid image pickupdevice according to Embodiment 1, successively readout signals arerespectively converted into digital signals and thereafter addition ofsignals can be performed also.

In this digital processing, since weighted addition using variableweights can be easily performed, as a result, addition of signals can beset as programmable, for example, as shown in FIG. 10. Using this methodenables, for example, the sensitivity to be varied corresponding todifferent ranges of light quantity.

Embodiment 6

The circuit of a solid image pickup device according to Embodiment 6 isshown in FIG. 11. A basic configuration and operation is the same asthose of Embodiment 2. The difference is that the capacity CFD of thecharge-voltage conversion part corresponding to the input part of asource follower is decreased and the number of readout capacity sets foreach signal output line is reduced from three sets to two sets so as toperform a transfer and a readout in two operations.

To be specific, the capacity CFD of the charge-voltage conversion partcorresponding to the input part of a source follower was set to theorder of 4fF to promote the sensitivity.

On the order of CFD=7fF, the charge conversion coefficient per electronin the input part is 23 μV/electron.

This embodiment was so designed as to set CFD to the order of 4fF and toset the charge conversion coefficient to 40 μV/electron.

In the prior art, a decrease in capacity value for enhancing thesensitivity led to the corresponding lowering of the dynamic range. Tobe specific, the handlable charge decreases to 57% and elevating thesensitivity and enlarging the dynamic range were hardly compatible.

In this embodiment, the power supply voltage is set to 5.0 volts and themaximum charge quantity accumulatable in a photodiode can be completelytransferred and read out by twice readout, and the signals read out bytwice readout were added on a common horizontal output line. As aresult, the sensitivity can be enhanced about double while ensuring thedynamic range.

Embodiment 7

The circuit diagram of three pixels of a solid image pickup deviceaccording to Embodiment 7 is shown in FIG. 12.

In this embodiment, three pixels are formed by connecting a sourcefollower amplifier as single amplification means, a selective switch anda reset switch to a photoelectric conversion part comprising threephotodiodes and three transfer switches.

Numeral 1301 denotes a power supply line; 1302: a reset switch line;1303: a selective switch line; 1305 a to 1305 c: photodiodes; and 1306:a transfer switch line.

This embodiment is characterized in that signals of individualphotodiode can be selectively read out by ON/OFF of the respectivetransfer switches, and on the other hand, signals of three photodiodesare added on the input terminal of the source followers by simultaneousON/OFF of three transfer switches. In case of adding three photodiodes,the signal amount increases as compared with the case of a singlephotodiode. Formerly, even when added, the signal amount was limited bythe signal amplitude on the input terminal of a source follower, but thereadout of a signal based on the charge accumulated for one unit ofaccumulation period by repeating several transfer operations enables alladded signals to be read out on application of the present invention.

Besides, in the present invention, a mode using the second or subsequentsignals without using the primary readout signal is selected to enableonly signals above a predetermined exposure quantity to be extracted.

In case of three-time readout or more, for example, selection andaddition of the second readout signal alone without addition of theprimary signal would enable signals above a certain exposure quantity tobe readily extracted without any digital computation.

Besides, since the power supply used for driving the pixel section canbe set to 3.3 volts, the pixel section can be driven together with an ADconverter by means of a single power supply. Furthermore, the pixelsection and the AD converter can be readily fabricated on one and thesame chip.

Besides, in the present invention, the N-type part and the P-type partcan also be changed to each other.

FIG. 13 shows a schematic illustration of a system using the above imagepickup device according to the present invention.

As shown in FIG. 13, the image ray incident through an optical system 71is focused on the solid image pickup device 72. By means of the solidimage pickup device 72, an light information is converted into anelectric signal. The electric signal is subjected to signal conversionprocessing in a signal processing circuit 73 by a predetermined methodsuch as white balance correction, gamma correction, brightness signalformation, color signal formation and contours correction and outputted.The signal subjected to signal processing is recorded orinformation-transferred by means of a recording system/communicationsystem 74 in an information recorder. The signal recorded or transferredis regenerated by means of a regenerative system 77. The solid imagepickup device 72 and the signal processing circuit 73 are controlled bya timing control circuit 75, while the optical system 71, the timingcontrol circuit 75, the recording system/communication system 74 and theregenerative system 77 are controlled by a system control circuit 76.Either independent readout only or addition/thin-out readout only can beselected by the timing control circuit 75.

Numeral 70 denotes a mechanical shutter for determining the exposuretime of the solid image pickup device 72, which is provided ifnecessary.

As described above, according to the embodiments of the presentinvention, the following effects can be obtained.

(1) Even for a lowered power voltage, a wide dynamic range of sensorsignal can be obtained.

(2) Owing to the above effect (1), a more minute MOS transistor can beused and downsizing of a pixel is enabled.

(3) Since a minute MOS transistor becomes usable, a common use with ahigh performance digital IC is enabled at a single power supply.

(4) Since a minute MOS transistor becomes usable, a single chip can beformed with a high performance digital IC at a single power supply.

(5) Enhanced sensitivity becomes realizable without deterioration ofdynamic range.

According to the present invention, it is possible to provide a solidimage pickup device having a lower power consumption and a smaller noiseratio than the conventional device, and a driving method therefor aswell as an image pickup system therefor.

Alternatively, it is possible without raising a power supply voltage ora reset voltage to provide a solid image pickup device capable ofperforming depletion transfer, and a driving method therefore as well asan image pickup system therefor.

1. A method of driving an image pickup device that includes a pluralityof pixels, the plurality of pixels including a plurality ofphotoelectric conversion units, a shared signal amplification transistorthat is shared by the plurality of photoelectric conversion units foramplifying signals generated in the plurality of photoelectricconversion units, an output node of the shared signal amplification unitbeing electrically connected to a vertical signal line, and a pluralityof charge transfer units for transferring electric charges from each ofthe plurality of photoelectric conversion units to an input node of theshared signal amplification unit, the method comprising: a firsttransferring step of transferring to the shared signal amplificationunit a portion of photoelectric signal charges accumulated in a firstphotoelectric conversion unit; a second transferring step oftransferring to the shared signal amplification unit a portion ofphotoelectric signal charges accumulated in a second photoelectricconversion unit; a first adding step of adding the signal chargestransferred in the first transferring step and the signal chargestransferred in the second transferring step at an input node of theshared signal amplification unit; a third transferring step oftransferring to the shared signal amplification unit a remaining portionof the photoelectric signal charges in the first photoelectricconversion unit, which remain from the first transferring step; a fourthtransferring step of transferring to the shared signal amplificationunit a remaining portion of the photoelectric signal charges in thesecond photoelectric conversion unit, which remain from the secondtransferring step; and a second adding step of adding the signal chargestransferred in the third transferring step and the signal chargestransferred in the fourth transferring step at an input node of theshared signal amplification unit.
 2. The method of driving an imagepickup device according to claim 1, wherein the first transferring stepand the second transferring step are carried out simultaneously, and thethird transferring step and the fourth transferring step are carried outsimultaneously.